Antibacterial Effect of Silver and Fluoride Compounds on Cariogenic Organisms

Objective: to investigate the antibacterial effects of silver- and fluoride-containing compounds used in caries management. Methods: Minimum inhibitory concentrations (MICs) of silver diammine fluoride (SDF), silver fluoride (AgF), silver nitrate (AgNO₃), ammonium fluoride (NH₄F) and sodium fluoride (NaF) were determined on Streptococcus mutans, Lactobacillus acidophilus and Actinomyces naeslundii. Serial (×2) dilutions from 2 ~ 400,000 µg•mL^-1 (limits depending on solubility) were prepared in a 96-well plate with BHI media adjusted to pH 7 or pH 5 to simulate neutral and cariogenic conditions. Tests with silver compounds were duplicated in the dark to investigate the effect of light. Results: Four-way analysis of variance (AoV) showed three main factors (agent, pH, species) to have highly-significant effects (p < 0.05) while light did not (p > 0.05). Dropping light and repeating as a three-way AoV, all factors remained significant as well as most interactions (p < 0.05). MIC(SDF) on the three bacteria ranged from 62 ~ 125 µg•mL^-1, similar to AgNO₃ (62 µg•mL^-1) and AgF (31 ~ 62 µg•mL^-1). For A. naeslundii and S. mutans, silver compounds had antibacterial effect at significantly lower concentration than the fluoride compounds (p < 0.001); NH₄F (313 µg•mL^-1) and NaF (500 µg•mL^-1) had significantly weaker (p < 0.05) effects at pH 7 than at pH 5. Increased antibacterial effect was found for F at the lower pH (p < 0.001) but not for Ag. MICs for L. acidophilus could not be determined at pH 5 due to inadequate growth, while at pH 7 there was no growth for NaF (2 µg•mL^-1) or NH₄F (20 µg•mL^-1), implying lower MIC values. Conclusions: Silver ions are antibacterial against oral microorganisms even at low concentration, while fluoride ion was only effective at relatively high concentration and affected by pH.